How to disable ttyAMA0 console on boot (Raspberry Pi)?

Running Ubuntu Server on a Raspberry Pi 3 B+ and I am unable to disable the console attached to the GPIO UART for the initial boot sequence. I am able to disable it in general execution (and have done so) but every restart it is enabled for the "Hit any key to stop autoboot" dialogue.

The board is to be placed in an autonomous vehicle, the accompanying board is constantly streaming telemetry through the interface. If it starts up first (it always does unless deliberately delayed) then the RPi is unable to boot because of that dialogue. Apart from the nuisance of the delayed start there is the obvious danger of the RPi being unable to recover in the event of a power cycle mid mission.

Ubuntu Server 18.04.4. LTS Raspberry Pi 3 B+ GPIO UART (ttyAMA0)

To disable the automatic start I did the following:

Removed console=ttyAMA0,115200 from /boot/firmware/nobtcmd.txt
Added dtoverlay=pi3-miniuart-bt to /boot/firmware/config.txt (or usercfg.txt, makes little difference)
Disabled [email protected]

The above steps have ensured its functionality in general use only, how do I prevent the initial service from running?

I had this issue resolved for 18.04 by bypassing grub completely by adding these lines to the [ALL] section of /boot/firmware/config.txt:

kernel=vmlinuz
initramfs initrd.img followkernel

I also had device tree commented out in the same file as well but not sure if it was necessary.

Here is a video which explains step by step how to prevent U-boot console from interrupting autoboot and sending debug messages on UART. I know links only answers are frowned upon, so here's a quick breakdown of a solution:

Install the dependencies

sudo apt install git make gcc gcc-aarch64-linux-gnu

Git clone the official u-boot repository. Alternatively you can git clone my fork of repository, where I already have the necessary changes for silent autoboot - but if you need the latest version, then you need to clone the official repository and make changes yourself.

git clone --depth 1 git://git.denx.de/u-boot.git

cd u-boot

Find the raspberry pi config files - they depend on the model, rpi_3_defconfig for Raspberry Pi 3, rpi_4_defconfig for Raspberry Pi 4 and so on.

Add the following lines to the end of the file:

CONFIG_BOOTDELAY=-2
CONFIG_SILENT_CONSOLE=y
CONFIG_SYS_DEVICE_NULLDEV=y
CONFIG_SILENT_CONSOLE_UPDATE_ON_SET=y
CONFIG_SILENT_U_BOOT_ONLY=y

The first line removes the boot delay, so autoboot will not be interrupted by messages sent on UART interface. The next four lines enable silent boot, so U-boot will not send any messages on UART itself, because the messages might in turn confuse your device. One more little thing left, set silent boot environmental variable. Change include/configs/rpi.h file

#define CONFIG_EXTRA_ENV_SETTINGS \
    "dhcpuboot=usb start; dhcp u-boot.uimg; bootm\0" \
    "silent=1\0" \
    ENV_DEVICE_SETTINGS \
    ENV_DFU_SETTINGS \
    ENV_MEM_LAYOUT_SETTINGS \
    BOOTENV

Now configure with

make rpi_3_defconfig

from repository main folder

And build with

make CROSS_COMPILE=aarch64-linux-gnu-

When build process finishes you will have a u-boot.bin file, which you need to rename and copy to Raspberry Pi SD card. Now your Raspberry Pi will not be disturbed by any messages on UART during boot. The UART functionality after boot will not be affected.

How to disable ttyAMA0 console on boot (Raspberry Pi)?

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